DC removal techniques for wireless networking

ABSTRACT

A wireless communication device (WCD) performs DC removal on a received signal using a coarse DC removal unit that removes relatively large DC components and a fine DC removal loop that removes residual DC components. The coarse DC removal unit can be implemented in a receiver, and the flue DC removal loop can be implemented in a modem. In addition, a coarse DC estimation loop implemented on the modem may be coupled to the coarse DC removal unit to update DC offset values stored in the DC removal unit. By storing coarse DC offset values locally on the receiver, DC removal can be achieved very quickly.

FIELD

This disclosure relates to wireless communication and, moreparticularly, to wireless local area network (WLAN) systems.

BACKGROUND

Wireless networks allow computing devices to share information andresources via wireless communications. Examples of computing devicesused in wireless networks include laptop or desktop computers, personaldigital assistants (PDAs), mobile phones such as cellularradiotelephones and satellite radiotelephones, data terminals, datacollection devices, personal digital assistants (PDAs) and otherportable and non-portable computing devices. One broad family ofstandards developed to facilitate wireless networking is set forth inthe IEEE 802.11 standard. The original IEEE 802.11 standard providesdata transfer rates of 1–2 Megabits per second (Mbps) in a 2.4–2.483Gigahertz (GHz) frequency band (hereafter the 2.4 GHz band). However, anumber of extensions to the original IEEE 802.11 standard have beendeveloped in an effort to increase data transfer rates.

The IEEE 802.11 standard (sometimes referred to as 802.11 wirelessfidelity or 802.11 Wi-Fi) is an extension of the IEEE 802.11 standardthat provides 11 Mbps transmission (with a fallback to 5.5, 2.0 and 1.0Mbps) in the 2.4 GHz band. The IEEE 802.11b standard utilizes binaryphase shift keying (BPSK) for 1.0 Mbps transmission, and quadraturephase shift keying (QPSK) for 2.0, 5.5 and 11.0 Mbps transmission.Complimentary code keying (CCK) techniques are also employed by 802.11bin order to achieve multi-channel operation in the 2.4 GHz band for the5.0 and 11.0 Mbps transmission rates.

The IEEE 802.11g standard is another extension of the IEEE 802.11standard. The IEEE 802.11g standard utilizes orthogonal frequencydivision multiplexing (OFDM) in the 2.4 GHz frequency band to providedata transmission at rates up to 54 Mbps. The IEEE 802.11g standard alsoprovides backwards capability with 802.11b networks. The IEEE 802.11astandard is an extension of IEEE 802.11 standard that utilizes OFDM in a5 GHz frequency band to provide data transmission at rates up to 54Mbps. These and other wireless networks have been developed. Additionalextensions to the IEEE 802.11 standard, as well as other WLAN standardswill likely emerge in the future.

Wireless networks may contain one or more access points that interfacewith wireless and/or wired networks. Access points may also interfacewirelessly with other access points to extend the geographical size ofthe wireless network. In addition, wireless routers may be used inwireless networks to perform data routing functions within the wirelesssetting. Sometimes, both wireless routers and access points are usedtogether to form a relatively large wireless network environment.

Wireless communication devices that support wireless networkingstandards may also support other communication standards, such asstandards commonly used for voice communications. The voicecommunication standards may be based on one or more of a variety ofmodulation techniques, such as frequency division multiple access(FDMA), time division multiple access (TDMA), and various spreadspectrum techniques. One common spread spectrum technique used inwireless voice communication is code division multiple access (CDMA)signal modulation. In CDMA, multiple communications are simultaneouslytransmitted over a spread spectrum radio frequency (RF) signal. Otherwireless communication systems may use different modulation techniques.For example, GSM systems use a combination of TDMA and FDMA modulationtechniques. These techniques are also used in other systems related toGSM systems, including the DCS1800 and PCS1900 systems, which operate at1.8 GHz and 1.9 GHz, respectively.

Due to constraints imposed by the wireless specifications, a signal of aWLAN system may need to be acquired more quickly than signals associatedwith most voice communication systems. For example, in a 802.11b WLANsystem, a data packet is preceded by an approximately 56 microsecond(μs) synchronization preamble. Of this 56 μs preamble, the wirelesscommunication device (WCD) may be allocated approximately 36 μs forsynchronizing a demodulator. Before the demodulator can be synchronized,however, the WCD may need to perform a number of tasks, including thetask of removing DC components from the received signal. Conventional DCremoval techniques used in voice communication typically providecontinuous monitoring of a received signal, estimation of the DC offset,and closed-loop feedback to facilitate DC removal. However, theconventional techniques commonly used in voice communication systems maylack sufficient speed to satisfy the time constraints imposed by a WLANsystem.

SUMMARY

In one embodiment, a wireless communication device (WCD) includes areceiver coupled to a modem. The receiver can quickly remove DC from ananalog baseband signal associated with a received packet by accessing anestimated DC offset value stored locally in memory of the receiver. Themodem may remove residual DC from a digital representation of thebaseband signal using a residual DC removal loop. In addition, the modemmay estimate the residual DC offset and update the memory of thereceiver so that the receiver can remove a more appropriate amount of DCfrom subsequently received analog baseband signals.

Various embodiments may be implemented in software, hardware, firmware,or any combination thereof. Additional details of various embodimentsare set forth in the accompanying drawings and the description below.Other features, objects and advantages will become apparent from thedescription and drawings, and from the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram illustrating a wireless communication systemin which wireless communication devices (WCDs) can implement DC removaltechniques.

FIG. 2 is a block diagram of a WCD depicted in FIG. 1.

FIG. 3 is a more detailed block diagram of a receiver and modem of theWCD depicted in FIG. 2.

FIG. 4 is a more detailed block diagram of a fine DC removal unit andcoarse DC estimator that form part of the modem depicted in FIG. 3.

FIG. 5 is a more detailed block diagram of the coarse DC removal unitthat forms part of the receiver depicted in FIG. 3.

FIG. 6 is a flow diagram illustrating DC removal techniques that can beimplemented in a WCD.

DETAILED DESCRIPTION

In general, this disclosure describes a wireless communication deviceconfigured to perform various signal processing tasks associated withwireless data communication. More specifically, a wireless communicationdevice (WCD) including a wireless LAN (WLAN) receiver may perform DCremoval very quickly on a packet-by-packet basis. The receiver of theWCD may store DC offset values estimated during the processing ofbaseband signals associated with a previously received packet. Theselocally stored DC offset values can be used by the receiver to quicklyremove relatively large DC offsets from an incoming baseband signalassociated with a subsequently received packet. As a result, DC removalcan be performed more quickly, and acquisition time may be reduced in aWLAN receiver.

During processing of a received baseband signal, a modem(modulator/demodulator) may estimate the DC offset, and use the estimateto update DC offset values stored locally on the receiver. In somecases, received signals may be processed according to one of a pluralityof gain states. In that case, DC offset values may be stored on thereceiver for each of the gain states. Thus, different DC offsets may beremoved at the receiver, depending on the gain state associated witheach received baseband signal. In other words, when a packet isreceived, it may be processed according to one of a plurality of gainstates, and the locally stored DC offset value associated with theselected gain state can be applied to the baseband signal associatedwith the packet in order to remove DC very quickly.

DC removal is generally necessary because significant DC offsets can beintroduced into the baseband signal during the mixing process (sometimesreferred to as down conversion). Moreover, in a WLAN setting, thereceived signal is generally not a constant signal; rather, receivedpackets are temporally separated. For this reason, DC removal presentssignificant challenges in a WLAN setting because temperature changes ofsignal processing components of the WCD, such as mixers, cansignificantly affect the amount of DC introduced into the basebandsignal. In addition, the amount of DC introduced by the system maychange significantly between the times when sets of packets arereceived. DC removal can be particularly challenging in settings thatimplement wireless standards for which the time allocated for DC removalis very short. The IEEE 802.11 , IEEE 802.11a, and IEEE 802.11gstandards are three examples of such a wireless standard.

As described in greater detail below, the WCD may implement coarse DCremoval techniques to remove relatively large DC components, as well asa fine DC removal techniques to remove residual DC components. Thecoarse DC removal can be achieved using memory, a DC digital-to-analogconverter (DAC) and a difference amplifier incorporated in a receiverchip. The fine DC removal can be achieved using a DC estimation andremoval loop on the modem chip. Estimations used to update coarse DCvalues stored in the receiver memory can also be calculated on the modemchip. Therefore, updates may be periodically sent from the modem chip tothe coarse DC removal unit on the receiver chip via a serial bus.

The techniques may exploit wireless networking practices that operateaccording to a resend-until-acknowledged protocol. Accordingly, areceived packet can be processed, and during the processing, the coarseDC removal unit can be updated. If the DC offset of the baseband signalassociated with the received packet is too large, then analog-to-digitalconverters in the modem may saturate during processing of the receivedpacket. In that case, processing of the packet will generally error-out.However, the packet will eventually be resent because the WCD will notreturn an acknowledgement to the sender for the packet that errored-out.

As a result, the sender will resend the packet, providing an opportunityto condition the incoming signal to remove a more appropriate portion ofthe DC component. Moreover, because the coarse DC removal unit isupdated to adjust the DC offset value during the processing of theinitial packet, the next time a copy of packet is received, the DCadjustments may be more likely to ensure that the baseband signal willfall within the range of the A/D converters. In some cases, the samepacket may need to be received a plurality of times before the coarse DCvalue is within a range that will allow the packet to be processedwithout saturating the A/D converters. Nevertheless, theresend-until-acknowledged framework should ensure that the packet willcontinue to be sent until the coarse DC value is acceptable.Importantly, using values stored locally at the receiver, coarse DCremoval at the receiver chip can be executed very quickly as required insome WLAN systems.

The techniques described below may yield a number of advantages. Forexample, the techniques may be used to significantly reduce the time ittakes for DC removal on a received baseband signal by storing DC offsetvalues locally on the receiver chip. Thus, coarse DC removal can beperformed very quickly, simply by selecting the appropriate stored DCoffset value and removing the value from the received baseband signalassociated with the received packet. This is particularly important forwireless networks such as IEEE 802.11a IEEE 802.11b and IEEE 802.11gnetworks where the amount of time allocated for signal synchronizationis extremely small. Accordingly, the techniques may be used to decreasethe time associated with DC removal.

Also, additional techniques outlined in greater detail below cansimplify the architecture by reducing the number of serial lines neededto transfer updated DC values between the modem and receiver. In anycase, the values can be accessed locally by the receiver, reducingconsumption of system bus clock cycles that would otherwise be necessaryfor communication with the modem. In addition, techniques for improvingthe estimation of the DC offset are also described, such thataccumulation in the residual DC removal loop occurs at different ratesduring and after RF training. Such techniques can improve the effectivesignal-to-noise ratio of the demodulated signal.

FIG. 1 is a block diagram illustrating a wireless communication system 2including a number of wireless communication devices 10A–10C,collectively referred to as wireless communication devices 10. Wirelesscommunication devices (WCDs) 10 may be any portable computing deviceconfigured to support wireless networking. Each device may be, forexample, a desktop or portable computer operating in a Windows™,Macintosh™, Unix, or Linux environment, a personal digital assistant(PDA) based on the Palm™, Windows CE, or similar operating systemenvironments for small portable devices, or other wireless device suchas a mobile radiotelephone, an interactive television, a wireless dataterminal, a wireless data collection device, an Internet kiosk, anetwork-ready appliance for the home environment, a wireless server, andthe like.

WCDs 10 communicate with one another in wireless communication system 2via wireless signals 8A–8D (hereafter wireless signals 8). Inparticular, WCDs 10 may communicate according to a wireless protocolsuch as the protocol defined by a wireless standard, e.g., one of thestandards in the IEEE 802.11 family of standards. Wireless signals 8 maybe sent to and from the respective WCDs 10 by wireless access points 11Aand 11B. The access points 11 may have wired connections to a network14, such as a local area network, a wide area network, or a globalnetwork such as the Internet.

In addition, one or more WCDs 10 within system 2 may be configured tosupport one or more voice communication standards. For example, one ormore base stations 4 may communicate voice data 9 to WCD 10A via voicecommunication techniques such as CDMA techniques, FDMA techniques, TDMAtechniques, various combined techniques, and the like. For example, oneor more of WCDs 10 may be designed to support one or more CDMA standardssuch as (1) the “TIA/EIA-95-B Mobile Station-Base Station CompatibilityStandard for Dual-Mode Wideband Spread Spectrum Cellular System” (theIS-95 standard), (2) the “TIA/EIA-98-C Recommended Minimum Standard forDual-Mode Wideband Spread Spectrum Cellular Mobile Station” (the IS-98standard), (3) the standard offered by a consortium named “3rdGeneration Partnership Project” (3GPP) and embodied in a set ofdocuments including Document Nos. 3G TS 25.211, 3G TS 25.212, 3G TS25.213, and 3G TS 25.214 (the W-CDMA standard), (4) the standard offeredby a consortium named “3rd Generation Partnership Project 2” (3GPP2) andembodied in a set of documents including “TR-45.5 Physical LayerStandard for cdma2000 Spread Spectrum Systems,” the “C.S0005-A UpperLayer (Layer 3) Signaling Standard for cdma2000 Spread SpectrumSystems,” and the “C.S0024 CDMA2000 High Rate Packet Data Air InterfaceSpecification” (the CDMA2000 standard), (5) the HDR system documented inTIA/EIA-IS-856, “CDMA2000 High Rate Packet Data Air InterfaceSpecification, and (6) some other standards. In addition, WCDs 10 may bedesigned to support other standards, such as the GSM standard or relatedstandards, e.g., the DCS1800 and PCS1900 standards. GSM systems employ acombination of FDMA and TDMA modulation techniques. WCDs 10 may alsosupport other FDMA and TDMA standards.

FIG. 2 is a block diagram of an exemplary WCD 10. As shown, WCD 10includes an antenna 20 coupled to a receiver 22, a modem(modulator/demodulator) 26 coupled to the receiver 22 via serial bus 29and analog transmission line 31, and a control unit 24 coupled to boththe receiver 22 and the modem 26. Control unit 24 may form part of modem26, but is illustrated separately for simplicity. In some cases, antenna20 may be coupled to a duplexer (not shown), which is in turn coupled toboth receiver 22 and a transmitter (not shown) that generates thewireless signals to be transmitted from the WCD 10. For simplicity,however, the duplexer and transmitter are not illustrated. In thisdisclosure, the term modem refers to a component or collection ofcomponents that can perform modulation, demodulation, or both modulationand demodulation.

Receiver 22 receives wireless RF signals in which data is modulatedaccording to a modulation scheme, such as the BPSK or QPSK modulationschemes typically implemented by devices compliant with the IEEE 802.11bwireless networking standard or the OFDM modulation scheme typicallyimplemented by devices compliant with the IEEE 802.11g wirelessnetworking standard. In either case, the received information comes inthe form of data packets encoded according to the modulation schemeused. Dividing the data into packets has several advantages includingenabling the sending device to resend only those individual packets thatmay be lost or corrupted during transmission.

Wireless networks typically operate according to aresend-until-acknowledged protocol in which the packets are resent toWCD 10 until WCD 10 acknowledges receipt of the packet. The techniquesoutlined below may exploit this resend-until-acknowledged framework byrecognizing that received packets may be used to adjust the stored DCoffset values, so that later received packets can be properly processed.In other words, the techniques recognize that if the DC offset valuesare not precise enough to ensure that a first packet can be processed,the resend-until-acknowledged framework ensures that another copy of thepacket will be sent again. Thus, the first packet can be used to adjustthe DC offset values so that the second copy of the packet can beproperly conditioned by the removing sufficient DC offsets, ensuringthat the packet is eventually received and processed correctly.Importantly, the time it takes to perform DC removal on any given packetcan be significantly reduced because DC offset information can be storedlocally on receiver 22.

Receiver 22 receives RF waveforms via antenna 20. Receiver typicallyconditions the received waveform, such as by filtering or scaling the RFwaveform and mixing the waveform down to baseband. For demodulation usedin IEEE 802.11b wireless networks, receiver 22 generates basebandsignals for I- and Q-components of the RF signal as is well known in theart. The I-component refers to the in-phase component of the complexwaveform, whereas the Q-component refers to the quadrature-phasecomponent of the complex waveform. In both cases, receiver 22 passes thebaseband signal for the respective I- or Q-components of the complexwaveform to modem 26 for demodulation. For example, I- and Q-basebandsignals can be sent from receiver 22 to modem 26 via analog transmissionline 31. Control unit 24 may send commands to receiver 22 and modem 26to control the processing of the received packet.

The techniques outlined below may be duplicated in hardware forprocessing of both the I- and Q-baseband signals. For simplicity,however, the following description describes the processing of abaseband signal associated with a received packet. It is understood,that a baseband signal associated with a received packet may correspondto either a I- or Q-baseband signal, and that similar circuitry may beduplicated to process both the I- and Q-baseband signals in parallel. Inaccordance with other standards, a single baseband signal may beprocessed as outlined below, or multiple baseband signals representingvarious components of the signal may be processed as outlined below.

Modem 26 demodulates the received baseband signal. Depending on theencoding scheme with the data rate being used, modem 26 may implementdemodulation techniques that exploit redundancy of the waveform used toencode the packet in order to increase processing speed. In any case,modem 26 demodulates the received packets in order to extract thepayload of the packets for presentation to the user of WCD 10.

FIG. 3 is a block diagram illustrating in greater detail oneimplementation of receiver 24 coupled to modem 26. As shown, receiver 24may include a gain state unit 32 that selects or stores a gain state forprocessing of a received packet. Gain state unit 32, for example, mayselect one of a plurality of gain states based on the power of the RFsignal associated with the received packet. Alternatively, gain stateunit 32 may receive and store an indication of a selected gain state.For example, power estimation techniques can be implemented in modem 26to estimate the power of the received RF signal so that signals can besent from modem 26 to gain state unit 32 via serial bus 29 to select theappropriate gain state.

The selected gain state coarsely defines the gain of one or moreamplifiers (not shown) and possibly mixer 34. For example, higher powersignals may be processed according to a lower gain state, whereas lowerpower signals may be processed according to a higher power gain state.In some embodiments, a single gain state may be used for all packets,and in other embodiments, any number of gain states can be implementedto improve the processing of signals of varying power levels. Asoutlined in greater detail below, DC estimation techniques can be usedto estimate DC offsets and update local memory of receiver 22 with DCoffset information for one or more gain states. In some cases, DC offsetinformation can be updated at the same time gain state selection signalsare sent from modem 26 to gain state unit 32.

Mixer 34 receives the RF signal and mixes it down to I- and Q-basebandsignals. For example, mixer 34 may implement a frequency synthesizerthat utilizes a local clock of WCD 10 as a timing reference. Thus, mixer34 may remove the RF carrier component of the received RF signal togenerate the baseband signals associated with the received packet.Again, although the following discussion describes the processing of abaseband signal associated with a received packet, it is understood thatduplicative circuitry may be implemented for both I- and Q-basebandsignals. As desired, receiver 22 may also include additional componentssuch as various filters, amplifiers and the like.

Coarse DC removal unit 36 stores values indicative an estimated DCoffset associated with the received baseband signal. For this reason,coarse DC removal unit 36 can quickly remove DC components from thebaseband signal associated with the received packet within the timeconstraints imposed by certain WLAN standards. If WCD 10 operatesaccording to a number of different gain states, coarse DC removal unit36 may store DC offset values associated with each of the gain states.In that case, coarse DC removal unit 36 may select the appropriate DCoffset value according to the gain state selected or stored in gainstate unit 32 in order to remove the appropriate amount of DC from thebaseband signal.

While the coarse DC removal unit 36 is removing a DC offset in thebaseband signal associated with the received packet, the baseband signalis being sent to modem 26 for demodulation. For example, the basebandsignals can be sent from receiver 22 to modem 26 via analog transmissionline 31. Receiver 22 and modem 26 may also be coupled together by aserial bus 29. Accordingly, receiver 22 and modem 26 may each include aserial bus interface 37, 39 to facilitate data transmission over serialbus 29.

Upon receiving a baseband signal associated with a received packet,modem 26 converts the signal to a digital representation (referred to asa digital baseband signal). In particular, analog to digital (A/D)converter 40 samples a received analog baseband signal and produces thecorresponding digital baseband signal. Fine DC removal unit 42implements a DC removal loop to remove residual DC from the digitalbaseband signal. In addition, coarse DC estimator 44 estimates theresidual DC offset associated with the baseband signal. After removingthe residual DC from the digital baseband signal, fine DC removal unit42 forwards the digital baseband signal to a digital variable gainamplifier (DVGA) 46. DVGA 46 can be used to scale the digital basebandsignal, either by amplifying or attenuating the digital values. Forexample, the DVGA may be implemented in lieu of, or in addition to theimplementation of various gain states. After scaling the digitalbaseband signal, DVGA 46 then forwards the scaled digital basebandsignal to demodulation unit 48 for demodulation and data extraction.

Coarse DC estimator 44 is implemented to estimate the residual DC offsetvalue associated with the received baseband signal. As mentioned, fineDC removal unit 42 also estimates the residual DC offset value and usesthe estimation in order to remove the residual DC from the digitalbaseband signal. Coarse DC estimator 44, in contrast, estimates theresidual DC offset value and uses the estimation to update the DCremoval value stored in coarse DC removal unit 36 on receiver 22. Thus,coarse DC estimator 44 may not affect the digital baseband signal forwhich the estimation is made. Instead, coarse DC estimator 44 adjuststhe DC removal value stored in coarse DC removal unit 36 so that the DCcomponents removed from baseband signals associated with subsequentlyreceived packets will be more adequate.

For example, in some cases, upon receiving a first packet the coarse DCremoval unit 36 will not remove enough DC to ensure that the packet canbe processed. In that case, if the residual DC is too large, AIDconverter 40 may saturate, thereby corrupting the digital basebandsignal. The subsequent processing of the digital baseband signal maycause an error, for example, during demodulation. Therefore, WCD 10 willnot return an acknowledgement to the sending device for that packet, andthe sending device will send another duplicate copy pursuant to thesend-until-acknowledged framework.

Furthermore, because coarse DC estimator 44 estimated the residual DCand adjusted the DC value stored in coarse DC removal unit 36, when theduplicate copy of the previously received packet is received andprocessed by receiver 22, coarse DC removal unit 36 will be more likelyto remove sufficient DC, ensuring that the packet can be processedwithout saturating A/D converter 40. In some cases, the same packet mayneed to be received a plurality of times before the coarse DC value iswithin a range that will allow the packet to be processed withoutsaturating the A/D converter 40. Nevertheless, theresend-until-acknowledged framework should ensure that the packet willcontinue to be sent until the coarse DC value is acceptable. In mostcases, the coarse DC value will quickly converge the an acceptable valueand packets will be processed quickly and efficiently.

Modem 26 may also perform one or more power detection techniques inorder to assess whether the current gain state is adequate. In thatcase, updates to the DC offset values stored locally on receiver 22 maybe made at the same time that signals are sent from modem 26 to receiver22 to set or adjust the gain state.

FIG. 4 is a more detailed block diagram of an exemplary implementationof fine DC removal unit 42 and coarse DC estimator 44. As illustrated,fine DC removal unit 42 receives the digital baseband signal from A/Dconverter 40 (FIG. 3) in the form of 10-bit values. Other sized A/Dconverters could also be used. A residual DC removal loop is implementedwithin fine DC removal unit 42 to remove residual DC components from thedigital baseband signal. For example, as illustrated, the 10-bit valuecan be shifted 50, scaled 54 and accumulated 56 to provide an ongoingestimate of the residual DC in the baseband signal. The residual DCoffset value is stored and accumulated in accumulator 56. Then, theaccumulated estimate of the residual DC offset can be rounded 58 andremoved from subsequent 10-bit values of the digital baseband signal asshown by adder 60. In other words, accumulator 56 stores the residual DCoffset correction value, which is used to remove DC from the digitalbaseband signal.

The residual DC offset correction value typically converges to thecorrect DC correction value after iterations of the loop. With a largerportion of the DC component removed, the baseband signal should modulatearound an amplitude of zero. Thus, by accumulating the signal amplitude,and removing the accumulated amplitude each iteration of the loop, theresidual DC can be removed from the baseband signal as shown in FIG. 4.

The gain (K) of the residual loop can be selected based on the desiredoperation of the loop. For example, a relatively large value of K, suchas K on the order of approximately 2⁽⁻³⁾ can result in very fastconvergence of the loop, e.g., less than approximately 10 μsec. As thevalue of K is increased, the time constant associated with loopconvergence may also increase, but the effective signal-to-noise ratioafter demodulation may be improved. Accordingly, the value of K may beselected or defined with these offsetting factors in mind.

In some cases, K may be initially set to be relatively large, such as avalue of K of approximately 2⁽⁻³⁾ during an RF training interval on theorder of 20 μsec to ensure quick convergence of the loop. Then, K may bechanged to a relatively small gain value such as approximately 2⁽⁻⁵⁾ orapproximately 2⁽⁻⁹⁾ after the RF training interval. In this manner,benefits associated with quick convergence of the loop and decreasedeffective signal-to-noise after demodulation may both be realized.

At some point following convergence of the residual DC correction loopwithin fine DC removal unit 42, control unit 24 (FIG. 2) may enable acoarse DC estimation loop within coarse DC estimator 44. For example,control unit 24 may send a coarse loop enable signal (coarse_en) toenable the estimation of the residual DC offset by coarse DC estimator44. The coarse DC estimation loop within coarse DC estimator may operatesimilar to the residual DC estimation loop within fine DC removal unit42. For example, as illustrated, the 10-bit residual DC estimate fromvalue received from fine DC removal unit 42 can be shifted 64, amplified66, and accumulated to provide an ongoing estimate of the residual DC.In the illustrated implementation, separate accumulators 68, 70, 72 areprovided for each of three possible gain states. As mentioned above,control unit 24 may be implemented within modem 26, or may be a separatecomponent as illustrated in FIG. 2.

Any number of gain states, and thus any number of accumulators may beused. As illustrated in FIG. 4, however, when multiple gain states aresupported, the accumulator associated with the current gain state can beenabled to form part of the coarse DC estimation loop. In other words,control unit 24 (FIG. 2) can enable the appropriate accumulator viaaccumulator enable signals (G1_en, G2_en, or G3_en). Thus, an estimationfor the DC offset can be accumulated for the particular gain stateassociated with the current baseband signal being processed. Multiplexer74 can be implemented to select the appropriate accumulator. Forexample, control unit 24 (FIG. 2) can provide a gain state select signal(Gstate_select) to multiplexer 74 to indicate which of the threeaccumulators are enabled. The output of multiplexer 74 can then berounded before updating the coarse DC removal unit 36 as describedbelow.

The gain (M) of the coarse DC estimation loop can be selected based onthe desired operation of the loop, and the operation parameters of DCdigital-to-analog converter (DC DAC) used in the coarse DC removal unit36 as outlined below. Typically, M may have a value betweenapproximately 1 and 2^((−5).) For example, an M value on the order ofapproximately 2⁽⁻³⁾ may be appropriate for most situations.

Coarse DC estimator 44 estimates the residual DC offset associated witha received baseband signal and updates coarse DC removal unit 36 (FIG.3) so that subsequently received baseband signals associated withsubsequently received packets will have the appropriate amount of DCremoved by receiver 22. Again, in the illustrated example, coarse DCestimator 44 accumulates a DC estimate specifically for a selected oneof a plurality of gain states. This accumulation process may continueuntil the packet has been completely forwarded from fine DC removal unit42 to DVGA 46 and demodulation unit 48. At that point, control unit 24(FIG. 2) may enable coarse DC estimator 44 to update coarse DC removalunit 36 (FIG. 3), such as by providing a DC update signal (DC_update) tocoarse DC estimator 44. Then, coarse DC removal unit 36 can send anupdate to coarse DC estimator 44 via serial bus 29. In otherimplementations, each accumulator may have its own dedicated serial bus.However, the implementation using the same serial bus for allaccumulators can save significant real estate for the circuit. In stillother cases, DC offset values for every gain state may be updated uponprocessing each packet.

FIG. 5 is a more detailed block diagram of an exemplary implementationof coarse DC removal unit 36 capable of receiving updates from thecoarse DC estintator44 illustrated in FIG. 4. In particular, coarse DCremoval unit 36 may periodically receive updated offset values for thedifferent gain states associated with the respective baseband signalbeing processed. For example, coarse DC removal unit 36 may receive anupdate from coarse DC estimator 44 via serial bus 29. Control unit 24may provide a signal indicative of the gain state associated with thereceived updates (Gstate_select) so tat multiplexer 80 can select andforward the update to the appropriate local memory associated with theappropriate gain state. In other words, DC removal unit 36 may maintainseparate DC offset values for each of the plurality of gain states. Forexample, the memory may be allocated for storing a DC value for a firstgain state 82, a DC value for a second gain state 84, a DC value for athird gain state 86, and so forth. These stored values may be digitalvalues as provided by coarse DC estimator. Also, in some cases theupdates may be provided to coarse DC removal unit 36 in receiver 22 atthe same time that control unit 24 or modem 26 updates or adjusts thegain state according to one or more power estimations.

When packets are received by receiver 22 and mixed down to basebandsignals, receiver 22 can perform DC removal very quickly because the DCoffset values are stored locally in memory 82, 84, 86. Moreover, thestored DC offset values may correspond to estimates that were made bythe coarse DC estimator 44 on modem 26 or a recently received packet.The DC offset values stored locally in memory 82, 84, 86, shouldconverge to values that will ensure that the packets can be processedwithout saturating analog-to-digital converter 40.

DC can be removed from analog baseband signals within receiver 22 byimplementing a DC digital-to-analog converter (DC DAC) 90. Inparticular, when an analog baseband signal associated with a receivedpacket is generated by mixer 34 and forwarded to coarse DC removal unit36 (FIG. 3), control unit 24 (FIG. 2) sends Gstate_select control signalto multiplexer 89. Multiplexer 89 selects and forwards the appropriateDC offset value, as determined by the gain state, to DC DAC 90. DC DAC90 converts the digital DC offset value to an analog representation ofthe DC offset. DC DAC 90 then applies the analog representation of theDC offset using difference amplifier 92 which removes the DC offset fromthe received baseband signal before sending the received baseband signalto modem 26 via analog transmission line 31. Importantly, because the DCoffset information is stored and updated locally on receiver 22, DCremoval can be performed very quickly as required by some WLAN systems.Thus, upon receiving a packet and selecting the gain state, DC removalcan be performed immediately at receiver 22 simply by accessing thestored DC value associated with the selected gain state.

FIG. 6 is a flow diagram illustrating one embodiment of DC removaltechniques that remove DC components from baseband signals on apacket-by-packet basis. As shown, when a packet is received (yes branchof 102), receiver 22 generates one or more baseband signals associatedwith the packet (103). In addition, receiver 22 removes an amount of DCfrom the baseband signal (104), such as by accessing a coarse DC valuestored in memory, converting the coarse DC value to an analog DC signaland subtracting the converted analog signal from the baseband signal.Receiver 22 may then send the baseband signal to modem 26, where thebaseband signal is converted to a digital baseband signal (105) andresidual DC is removed (106). In addition to removing the residual DC,modem 26 estimates the residual DC (107) so that the coarse DC valuestored on the receiver can be updated at a later time. Modem thendemodulates the baseband signal (108) and determines whether the packetwas received and processed without error. If modem 26 identifies anerror for the packet (yes branch of 109), WCD 10 does not send anacknowledgement to the sending device (110). In that case, the sendingdevice should resend the packet. If modem 26 does not identify an error,however, WCD 10 sends an acknowledgement to the sending device toindicate that the packet was received (111).

As mentioned, when modem is removing the residual DC (106), it may alsoestimate the DC offset (107). Then, after demodulation, modem 26 sendsan updated coarse DC value to receiver 22 (112). In this manner, when asubsequent packet is received (yes branch of 102), receiver 22 generatesone or more baseband signals for the subsequent packet (103) and removesan amount of DC from the baseband signal (104) according to the updatedcoarse DC value provided by modem 26. In other words, the residual DCestimated during the processing of a baseband signal associated with afirst packet, will be removed by the receiver 22 when a subsequentbaseband signal associated with a second packet is processed. Similarly,the residual DC estimated during the processing of a baseband signalassociated with the second packet, will be removed by the receiver 22when a subsequent baseband signal associated with a third packet isprocessed, and so forth.

In some cases, an error may occur for a received packet (yes branch of109) specifically because not enough DC was removed by receiver 22.However, WCD 10 will not send an acknowledgment in that case (110).Thus, another copy of the packet should be received in the near futurebecause of the resend-until-acknowledged framework. Moreover, becausethe coarse DC value stored in receiver 22 is updated (112), removal ofcoarse DC from a baseband signal associated with a later received copyof the same packet may be sufficient to ensure that the second copy canbe processed without causing an error. In some cases, the same packetmay need to be received a plurality of times before the coarse DC valueis within a range that will allow the packet to be processed withoutsaturating the A/D converter. Nevertheless, theresend-until-acknowledged framework should ensure that the packet willcontinue to be sent until the coarse DC value is acceptable.Importantly, coarse DC removal at the receiver can be executed veryquickly as required in some WLAN systems.

Various techniques for performing DC removal have been described asbeing implemented in hardware. Example hardware implementations mayinclude implementations within a DSP, an application specific integratedcircuit (ASIC), a field programmable gate array (FPGA), a programmablelogic device, specifically designed hardware components, or anycombination thereof. In addition, various other modifications may bemade without departing from the spirit and scope of the invention.Accordingly, these and other embodiments are within the scope of thefollowing claims.

1. A wireless communication device comprising: a receiver including amemory that stores a coarse DC offset value, wherein the receiverremoves a DC offset from a baseband signal according to the stored DCoffset value; and a modem coupled to the receiver, wherein the modemestimates a residual DC offset associated with the baseband signal andupdates the stored DC offset value in the receiver, wherein the coarseDC offset value is a digital value, wherein the receiver removes the DCoffset from the baseband signal according to the stored DC offset valueby converting the digital value to an analog value and removing theconvened analog value from the baseband signal.
 2. The wirelesscommunication device of claim 1, wherein the memory stores a pluralityof coarse DC offset values associated with a plurality of gain state,wherein the receiver removes the DC offset from the baseband signalaccording to a stored DC offset value associated with a selected gainstate, and wherein the modem estimates the residual DC offset associatedwith the baseband signal and updates the stored DC offset value in thereceiver for the selected gain state.
 3. The wireless communicationdevice of claim 1, wherein the modem includes a fine DC removal unitthat removes a residual DC offset from a digital representation of thebaseband signal.
 4. A wireless communication device comprising: a modemcoupled to a receiver, wherein the modem estimates a DC offset value fora baseband signal associated with a first received packet and updateslocal memory of the receiver with the estimated DC offset value; andwherein the receiver removes a DC offset from a baseband signalassociated with a second received packet according to the estimated DCoffset value stored in local memory.
 5. The wireless communicationdevice of claim 4, wherein the first received packet is processedaccording to a selected one of a plurality of gain states, wherein theupdated local memory is associated with the selected gain state, andwherein the second received packet is processed according to theselected gain state.
 6. The wireless communication device of claim 5,wherein the modem estimates a DC offset value for a baseband signalassociated with a third received packet, wherein the third receivedpacket is processed according to a different gain state than the firstreceived packet, and wherein the modem updates local memory of thereceiver with the estimated DC offset value for the baseband signalassociated with the third received packet, wherein the local memoryupdated with the estimated DC offset value for the baseband signalassociated with the third received packet is associated with thedifferent gain state; and wherein the receiver removes a DC offset froma baseband signal associated with a fourth received packet according tothe estimated DC offset value in local memory associated with thedifferent gain state.
 7. An apparatus comprising: a receiver thatincludes a gain state unit to store an indication of a selected gainstate, a mixer to convert a received RF signal to a baseband signal, anda coarse DC removal unit to remove a DC offset from the baseband signal;and a modem coupled to the receiver that includes an analog to digitalconvener to convert the baseband signal to digital sample; a fine DCremoval unit to remove a residual DC offset from the digital samples, acoarse DC estimator to estimate the residual DC offset and update thecoarse DC removal unit, a digital variable gain amplifier to scale diebaseband samples, and a demodulation unit to demodulate the basebandsamples.
 8. The apparatus of claim 7, wherein the coarse DC removal unitincludes a memory to store DC offset values for a plurality of gainstates, wherein the DC removal unit removes the DC offset from thebaseband signal according to a stored DC offset value corresponding tothe selected gain state used to process the baseband signal.
 9. Theapparatus of claim 7, wherein the fine DC removal unit removes theresidual DC offset from the digital samples using a DC removal loop thataccumulates estimates of DC offset values for the digital samples andremoves a value associated with the accumulation from the digitalsamples.
 10. The apparatus of claim 7, wherein the coarse DC estimatorestimates the residual DC offset by accumulating DC offset valuesassociated with the digital samples, and updates the coarse DC removalunit via a serial bus interface that connects the receiver the modem.